The main objectives of the study are a) to assess the extent of worsening/relapse upon discontinuation of imipramine and to test the hypothesis that maintenance on the drug protects from such reversal in a total experimental sample of 100 subjects with panic disorder and agoraphobia who have shown good and stable response to 6 months of systematic open treatment prior to randomization to the 12 month prospective placebo controlled discontinuation/maintenance phase of the study; b) An evaluation of late relapse/reversals achieved by extending the observation period of the double-blind placebo substitution condition to 24 months; c) A controlled assessment of relapse within the 12 month period following discontinuation of 1 year maintenance treatment d) A follow-up for 3 to 4 years from initial randomization of study completers and relapsers who are retreated as well as subjects who drop out from various stages of the double-blind protocol in order to gather systematic longitudinal and clinically relevant data specifically as they relate to relapse after varying lengths of imipramine maintenance treatment. The methods employed include an initial open treatment phase for 6 months. Patients who show a good response to treatment are randomized to double-blind treatment to either placebo or continuation of medication for 12 months. A second randomization then occurs; the placebo group continues on placebo and the medication group is randomized again to either placebo or continuation of the medication.